1. Field of the Invention
This invention relates to a thermal print head which is used in a facsimile machine or printer for example. More specifically, the present invention relates to a line-type thermal print head which has a line of heating dots with a length for sufficiently covering the entire width of paper being printed.
2. Description of the Prior Art
A line-type thermal print head is conventional and disclosed for example in Japanese Utility Model Laid-open No. 60-141256 and 63-92751. The name "line-type thermal head" is given because it includes a multiplicity of heating dots disposed in a line arrangement.
Specifically, the line-type thermal head disclosed in either of the above-described Japanese publications comprises a head circuit board on which the line of heating dots is formed to extend longitudinally thereof. The head circuit board further carries a longitudinal array of drive IC's for driving the heating dots, and a conductor pattern which includes connection terminals arranged substantially over the entire length of the head circuit board.
The line-type thermal head further comprises a metallic support member for supporting the head circuit board thereon and for dissipating the heat therefrom during operation. A flexible connector board carrying a conductor pattern is partially overlapped on the head circuit board. Further, a presser cover arranged above the connector board is fixed to the support member by a plurality of bolts screwed in the support member for covering the array of drive IC's for protection. The underside of the presser cover is provided with a groove which receives a separate elastic rod for pressing the connector board into contact with the head circuit board when the bolts are tightened up.
According to the above-described arrangement, the connection terminals are distributed substantially over the entire length of the head circuit board. Thus, the presser cover need be bolted to the support member substantially over the entire length of the head circuit board, so that the presser cover and the support member cannot linearly expand independently of each other under heating. This results in that the thermal head is subjected to thermal bending due to the so-called "bimetal phenomenon", consequently causing deterioration of the printing quality.
Further, the conventional thermal print head is also disadvantageous in the following points.
(1) The separate elastic rod need be fitted in the groove of the presser cover at the time of assembling the thermal head. Thus, it is relatively time-taking to assemble the thermal head, which results in increase of the cost. Further, the use of the elastic rod adds to the total number of required components.
(2) The presser cover is a thick and rigid member which must be made by extrusion for example. Thus, the cost for making the presser member is relatively high. Further, the use of the presser cover adds greatly to the overall weight of the thermal head.
(3) To realize uniform contact between the flexible connector board and the head circuit board, the tightening force of all the bolts must be strictly controlled. Such necessity also adds greatly to the cost of the thermal head.
U.S. Pat. No. 4,963,886 to Fukuda et al discloses another line-type thermal print head wherein the problematic thermal bending is prevented or reduced by arranging the connection terminals of the head circuit board in a limited central portion thereof while allowing the support member and the presser cover to linearly expand independently of each other at both ends of the thermal head. Although the problem of thermal bending is effectively prevented, the thermal head of this U.S. patent stills has the disadvantages (1)-(3) discussed above.